Arrangement for controlling the inductances of loop antennas



June 2, 1942. I BE'R-GTOLD 2,284,719

ARRANGEMENT FOR CONTROLLING THE INDUCTANCE OF A LOOP ANTENNA Filed Oct. 24, 1 939 2 Sheets-Sheet l Inventor Gtforncgl June 2, 1942. F. BERGTOLD 2,284,719

ARRANGEMENT FOR CONTROLLING THE INDUCTANCE OF A LOOP ANTENNA Filed Oct. 24, 1939 2 Sheets-Sheet 2 Q Enventor rfzficr (Iftorneg Patented June 2, 1942 ARRANGEMENT FOR CONTROLLING THE INDUCTANCES OF LOOP ANTENNAS Fritz Bcrgtold, Munich, Germany, assignor to Tclefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application October 24, 1939, Serial No. 300,938 In Germany September 30, 1938 4 Claims.

This invention relates to loop or frame antennas and more particularly to an improved means for varying the inductance of a loop antenna to permit the loop circuit to be accurately tuned or balanced after the loop has been completely assembled.

The inductance of a loop antenna depends on the length of the winding and arrangement of the winding. However, in practice it is not possible to duplicate a given arrangement in exactly the same manner. Therefore, it is necessary to adjust the inductance of each of the loops, and this adjustment should be as gradual as possible. It has been suggested to place adjustable loading coils in series or in parallel with the loop. If the step-like control which results from the use of a tapped winding is to be avoided, and if a substantial variation of inductance is desired the inductance of a series loading coil must be equal to a considerable fraction of the loop inductance, while the inductance of a parallel loading coil must be low, so that it represents a substantial load on the loop. The reason for this lies in the fact that where a gradual control is utilized it is always only a fraction of the total inductance that can be varied.

This invention will be better understood from the following description when considered in connection with the accompanying drawings. Its scope is indicated by the appended claims.

Referring to the drawings, Figure 1 is a general view of a loop antenna which includes an adjusting ring in accordance with this invention; Figure 2 is a perspective view of one form of adjusting ring; Figure 3 is a view of a practical system using an adjusting ring of the type illustrated in Figure 2; Figure 4 is an alternative type of adjusting ring; and Figure 5 illustrates the application of a ring of the type shown in Figure 4 to a loop antenna.

In accordance with the invention a ferrous adjusting ring is placed around the loop so that a variable iron path is made available for a part of the natural field of the frame. Examples of construction embodying the idea of the invention are shown in the Figures 1 to 5. Figure 1 indicates the general arrangement. The direction finder loop P is surrounded by an annular member E whose magnetic resistance can be varied.

Figure 2 shows an adjusting ring arrangement formed of three ferrous semiannular members R1, R2 and R3. The ferrous magnetic semiannular parts R1, R2 and R3 shown in Figure 2 surround the loop tube. The semiannular parts R2 and Rs are mounted so that they may be turned in common with respect to the annular part R1. Alternatively R2 and R3 are stationary and R1 is adapted to be turned. The rings have spacer disks of small thickness placed between each other so that at highest inductivity ferrous magnetic interspaces of finite value still remain. This measure is necessary to render ineffective slight variations in the relative positions, which may later present themselves.

Figure 3 shows the way in which the three semiannular parts of Fig. 2 can be arranged. The ring parts R2 and R3, which are stationary in this case, are arranged in casings G2 and G3 while the rotatable ring part (R1 in this case) is placed in the casing G1. The casing G1 has a fluted border and a fastening means, for instance in the form of a set screw. The arrangement shown in Fig. 3 may be made of an insulating mass and its purpose is to hold the adjusting rings and to close up the loop shield at the place at which it is electrically open.

In Fig. 4 the adjusting arrangement consists of a U-shaped piece U which contains iron and which surrounds the loop tubing, as before, and an armature A. The armature A is rotatable about the axis XX indicated in dot and dash lines whereby the magnetic path can be varied.

Fig. 5 shows the manner in which an adjusting ring arrangement similar to that shown in Fig. 4 can be built into the loop support. The

U-shaped part U surrounds the loop turns W at the bottom thereof. The frame connections Z are brought downwards at the side. The cover S of the loop support has a shaft L inserted thereinto carrying at one hand a handle H and on the other hand the rotatable armature A, so that the permeability of the adjusting ring may readily be adjusted. Alternately the shaft L may be adapted to move the armature away from the U-shaped part, without rotation.

I claim as my invention:

1. A device of the character described comprising a loop antenna having a metallic shield which is electrically open circuited at a point on the circumference thereof, an adjusting ring enclosing said loop at said point, said ring comprising at least two semiannular members of magnetically permeable material, and means for rotating one of said members with respect to the other to vary the magnetic permeability of said ring.

2. A device of the character described, comprising a loop antenna having an annular metallic shield enclosing the turns of said loop, said shield being electrically open circuited ata point on the circumference thereof, insulating means connecting the adjacent ends of said shield, a magnetically permeable element embedded in said insulating means and enclosing said loop at said point, and means for varying the permeability of said element to tune said loop to resonance at an operating frequency.

3. An arrangement for controlling the inductance of a direction finding loop, said loop having a shielding member electrically open circuited 10 at a point on the circumference thereof comprising an adjusting ring having separate relatively movable portions enclosing the turns of said loop and mounted in relatively movable FRITZ BERGTOLD. 

